1. Field of the Invention
This invention relates to a method for the disposal of waste gas and to apparatus therefor. More particularly, this invention relates to a method for the disposal of waste gas emanating from the process of production of chemicals to ensure complete oxidation of hydrocarbons, carbon monoxide, and other inflammable organic compounds contained in the waste gas and to apparatus used for the disposal of the waste gas.
2. Description of Prior Arts
Waste gases which occur at plants for production of chemicals contain hydrocarbons of 1 to 6 carbon atoms such as methane, ethane, ethylene, propane, and propylene, carbon monoxide, and inflammable organic compounds such as organic acids, aldehydes, esters, and alcohols and, moreover, entrain substances of offensive odor in large amounts. It is irrefutabley undesirable that such inflammable compounds should be left diffusing into the air. Particularly, removal of offensive odor from the waste gas has been in strong demand. A variety of processes developed for the elimination of such compounds have been proposed to the art.
The conventional waste-gas disposal processes for the removal of such offensively smelling substances and other noxious substances include those represented by the flow sheets of FIG. 1 and FIG. 2, for example. In the process of FIG. 1, a waste gas containing inflammable compounds and supplied via a conduit 1 is forwarded through a booster fan 2 optionally installed to suit the occasion, then heated in a heat exchanger 3, and forwarded via a conduit 4 to a heater 5, there to be heated with the heat supplied by an external heat source. The waste gas thus heated is sent to a catalytic oxidation reactor 6 packed with a catalyst having a noble metal such as platinum dispersed and carried on an activated alumina. In this reactor, the waste gas is completely burnt. The combustion gas is subsequently fed via a conduit 7 to a heat recovery unit 8, there to be deprived of the heat. The combustion gas consequently cooled through loss of heat is fed to the heat exchanger 3 and used therein for the purpose of heating the waste gas delivered via the conduit 1. Finally, the gas is released into the air through a stack 9. When the quantity of the heat generated by the waste gas supplied to the aforementioned catalytic-oxidation reactor 6 is too large, it is diluted with the air introduced through an air supply fan 10 for dilution of the heat generated by the waste gas.
In the process of FIG. 2, a waste gas containing inflammable compounds and fed in via a conduit 11 is forwarded via a booster fan 12 optionally installed to suit the occasion, then heated in a heat exchanger 13, and delivered via a conduit 14 to a catalytic-oxidation reactor 16 packed with a catalyst having a noble metal such as platinum dispersed and carried on an activated alumina. In the reactor 16, the waste gas is completely burnt. The combustion gas is subsequently fed via a conduit 17 to a heat recovery unit 18, there to be deprived of the heat. The combustion gas which is cooled by the loss of its heat is supplied to a heat exchanger 13, there to be utilized for the purpose of heating the waste gas delivered via the conduit 11. Thereafter, the gas is released into the air through a stack 19. In this case, part of the combustion gas emanating from the catalytic-oxidation reactor 16 is forwarded through a branched conduit 20 and mingled with the heated waste gas being forwarded by a fan 21 through the conduit 14 so as to elevate further the temperature of the waste gas and enhance the thermal efficiency of the process. Part of the combustion gas forwarded through the branched conduit 20 may be supplied via a branched conduit 22 to the heat exchanger 13. When the quantity of the heat generated by the waste gas fed to the catalytic-oxidation reactor 16 is too large, the gas emanating from the heat recovery unit 18 may be partly circulated via a conduit 23.
In either case, since the waste gas containing hydrocarbons, carbon monoxide, and other inflammable organic compounds is catalytically oxidized completely to an extent of being completely freed of noxiousness within the catalytic-oxidation reactor, this reactor is provided with a varying method for preventing possible short pass of the reaction gas.
Generally, when a waste gas containing inflammable organic compounds, particularly paraffinic hydrocarbons, is disposed by a process using a catalyst intended to ensure complete oxidation of such organic compounds, the preconditions for effective operation of this process are as follows:
(1) The gas temperature at the outlet of the catalyst bed should be substantially constant. It is said that the catalyst withstands heat to 700.degree. to 720.degree. C. It is not desirable to operate the catalytic-oxidation reactor at temperature conditions such that the gas temperature at the outlet of the catalyst bed falls short of reaching 650.degree. C., because the combustion of paraffinic hydrocarbons, particularly propane, becomes incomplete under such temperature conditions. The operation, therefore, should be carried out so that the outlet gas temperature will remain on the average level of about 680.degree. C.
(2) By reason of the catalytic activity, the gas temperature at the inlet to the catalyst bed should be not less than 250.degree. C.
(3) The heat generated by the inlet gas to the catalytic-oxidation reactor should be controlled so that the temperature of self elevation in the catalyst bed will remain within a range of 430.degree. C. (680.degree.-250.degree. C.). Notwithstanding this requirement, since the quantity of the heat generated by the waste gas under treatment is large and the amplitude of the change in the quanity of this heat is also large, the process itself should be such as to provide treatment of the waste gas stably.
The inventors made a study in search for a method of waste gas disposal with the foregoing preconditions in mind. They have consequently acquired a knowledge that the following points are indispensable to fulfilment of the preconditions.
(A) Maximum recovery of heat
An effort to lower the temperature of the treated waste gas to be released into the air to the fullest possible extent has its own limit from the economic point of view. It is, therefore, important that the increase in the volume of the waste gas (due to introduction of fresh air, for example) should be avoided by all means. To ensure the maximum recovery of heat, the process should be designed so that the temperature of the gas released into the air will be kept constant as much as possible against possible variation in the quantity of the heat generated by the incoming waste gas.
(B) Reuse of waste gas after treatment.
In the waste gas which has undergone the whole process, virtually all the inflammable substances have been removed and the oxygen concentration has been lowered as well. This waste gas, therefore, if desired to be utilized as an inert gas in various applications such as, for example, in sealing tanks which store dangerous substances.
For the process of FIG. 1 to continue to satisfy the aforementioned preconditions and, at the same time, fulfil the requirements of (A) and (B), there is inevitably entailed a disadvantage that the process necessitates the service of the heater 5 or an inconvenience that the volume of the waste gas is increased by the introduction of air for the dilution of the heat generated by the waste gas. The waste gas emanating from the catalytic-oxidation reactor 6, on reaching the heat recovery unit 8, contributes its heat to the generation of steam. It is further utilized for heating the incoming waste gas before it is finally released into the air. When the quantity of the heat generated by the incoming waste gas is large, the outlet gas temperature of the reactor is proportionally high. For effective control of the outlet gas temperature, therefore, air must be introduced from the ambience into the waste gas. This addition of air has an inevitable effect of increasing the total volume of the waste gas and consequently increasing the quantity of the heat entrained by the gas. It also brings about a disadvantage that the constancy of the oxygen concentration in the outlet gas is impaired and the reuse of the treated waste gas is inconvenienced. Conversely when the quantity of the heat generated by the incoming waste gas is low, the inlet gas temperature of the catalytic-oxidation reactor must be elevated so as to maintain the outlet gas temperature of the catalytic-oxidation reactor constant at the specified level. This necessitates the service of the temperature elevation heat exchanger 5. To ensure complete oxidation of the inflammable compounds in the waste gas, the outlet gas temperature of the catalytic-oxidation reactor must be maintained at a level of about 680.degree. C. and, for that purpose, the inlet gas temperature of the catalytic-oxidation must be elevated proportionally. Thus, the heat source used for the process is required to be capable of supplying heat of so much higher temperature. When the quantity of the heat generated by the waste gas is such as to involve a temperature of 370.degree. C. for self elevation, for example, the outlet temperature of the catalystic-oxidation reactor cannot be maintained at 680.degree. C. unless the inlet temperature is adjusted to 310.degree. C. Consequently, the heat source must be capable of supplying heat of about 350.degree. C. In ordinary chemical plants, it is difficult to find a heat source of a high temperature of the order of 350.degree. C. In actuality, therefore, such chemical plants have to depend on fuel of some sort or other to supplement their meager heat sources.
In the process of FIG. 2, in order that the process may satisfy the aforementioned preconditions and, at the same time, fulfil the requirements of (A) and (B), the apparatus for the waste gas disposal is designed on the basis of the conditions prevailing while the quantity of the heat generated by the incoming waste gas is at its peak. To be specific, in order to maintain the outlet gas temperature of the catalytic-oxidation reactor at 680.degree. C., the apparatus is designed so that the outlet temperature of the heat exchanger 13 remains at 680.degree. C. minus (the temperature of self elevation while the quantity of the heat generated is at its peak). If this temperature is not more than 250.degree. C., part of the outlet gas from the catalytic-oxidation reactor is diverted to be utilized for elevating the inlet temperature of the catalytic-oxidation reactor. This diverted gas is only effective in elevating the inlet gas temperature of the catalytic-oxidation reactor and not effective at all in elevating the outlet gas temperature of the same reactor. When the quantity of the heat generated by the incoming waste gas is lowered, therefore, it is no longer possible to maintain the outlet gas temperature of the catalytic-oxidation reactor at 680.degree. C. by suitably increasing or decreasing the volume of the diverted gas. It becomes necessary to elevate the outlet temperature of the heat exchanger 13. For this purpose, it is necessary to bypass the heat recovery unit 18 and divert part of the outlet gas of the reactor 16 to the heat exchanger 13 enough to elevate the temperature of the incoming waste gas. As an inevitable consequence, the temperature of the treated waste gas flowing through the stack 19 is elevated and the ratio of heat recovery is heavily degraded.
When the waste gas containing hydrocarbons, carbon monoxide, and other inflammable organic compounds is to be completely oxidized by the catalytic reaction, the apparatus is desired to be operated so that the interior temperature of the reactor, particularly the outlet gas temperature of the catalytic-oxidation reactor may be maintained constantly at an average level of about 680.degree. C. In this case, it is desirable even from the economic point of view to minimize the cross section of the reactor by using a monolithic catalyst of low pressure loss, i.e., a catalyst possessed of the so-called honeycomb structure.
This monolithic catalyst is generally obtained by forming an infusible refractory inorganic carrier such as, for example, cordielite or mullite in the shape of a honeycomb structure, coating this honeycomb substrate with a thin layer of activated alumina to impart a high specific surface area thereto, and depositing a noble metal such as platinum or palladium and an oxide of a heavy metal such as cobalt or manganese on the thin layer. The monolithic catalyst in popularly used as a catalyst capable of completely oxidizing hydrocarbons and carbon monoxide.
Because of the shape of the monolithic catalyst, it is difficult to fill the reactor completely with the monolithic catalyst. Consequently, an empty space occurs inevitably between the catalyst bed and the inner wall of the reactor. Insertion of partition plates is an effective measure generally adopted for perfect isolation of the empty space and preclusion of gas leakage. When the interior temperature of the reactor is relatively low, such partition plates may be welded to provide airtight seal of the interior of the reactor. When the outlet gas temperature of the catalyst bed reaches a high level of about 680.degree. C., however, the method of air-tight sealing by the welding of partition plates cannot be adopted. To avoid the distortion of the reaction due to the thermal expansion of the material of the reactor, the reactor must inevitably be designed so that the substrate for supporting the catalyst bed is given amply allowance for free thermal expansion or contraction instead of being directly welded to the inner wall of the reactor. As a good example befitting this purpose, there may be cited a reactor in which a catalyst bed designed in itself to preclude possible short pass of gas and a catalyst bed support attached fast to the inner wall of the reactor and designed in itself to preclude possible short pass of gas are so disposed relative to each other that they may maintain surface contact smooth enough to permit mutual expansion or contraction and obviate the distortion of the catalyst bed due to thermal expansion. Otherwise, because the air-tight sealing by welding is difficult to materialize, the only effective method may be dividing each of the partition plates into small sections and disposing these small sections side by side to warrant full absorption of thermal expansion. This method inevitably entails a complication that the adjoined edges of the sections should be prevented from producing gaps therebetween. In any event, it is next to impossible to prevent the untreated waste gas from short passing the empty space which is inevitably produced owing to the convenience of structural design of the reactor. Thus, the efficiency of the waste gas disposal is degraded or lowered to a point where perfection of the waste gas disposal can hardly be expected.
A method for preventing the empty space in the reactor from involving the short pass of the untreated waste gas by filling the empty space with a packing material such as refractory glass or asbestos has been in vogue for some time. Even if the empty space is amply filled with the packing material under the atmospheric temperature condition, the air-tightness offered by the packing inevitably becomes insufficient at the elevated temperatures of the operation. This method, therefore, has much to be desired.
An object of this invention, therefore, is to provide a method for the disposal of waste gas which permits maximum recover of heat and apparatus for use in this method.
Another object of this invention is to provide a method for the disposal of waste gas which permits effective exploitation of the waste gas resulting from the disposal and apparatus for use in this method.
Yet another object of the present invention is to provide a method for the disposal of waste gas by use of a catalytic-oxidation reactor entailing no short pass of the waste gas and apparatus for use in this method.